1. Technical Field of the Invention
The present invention relates in general to the telecommunications field and, in particular, to a flexible Radio Link Control (RLC) protocol for a mobile communications system.
2. Description of Related Art
When data is conveyed between nodes in a telecommunication network, certain algorithms are used to recover from the transmission of erroneous data and the loss of data on the transmission links between the nodes. An algorithm commonly used to recover from the transmission of erroneous data is referred to as an Automatic Repeat Request (ARQ) protocol.
The existing ARQ protocols include two peer entities that communicate with each other over transmission links. Each such entity includes a receiver and a sender. The units of data conveyed between the peer entities are commonly referred to as Protocol Data Units (PDUs). The ARQ protocols include certain rules for sending and receiving PDUs, as well as rules for the structure of the PDUs.
The receiver can inform the sender about which PDUs were correctly received (i.e., receiver acknowledges correctly-received PDUs) and/or which PDUs were incorrectly received. When the sender receives this information, it retransmits the “lost” PDUs. In other words, an ARQ protocol is a set of rules that allow the use of efficient retransmission mechanisms between a sending side and receiving side in a communication system. These rules specify, for example, how and in what form the PDUs are to be constructed so that the receiving side can interpret the conveyed PDUs correctly and respond to them accordingly.
Three main types of information elements (PDUs) can be transferred between two ARQ peer entities: user data; error recovery control data; and common control data. These three types of PDUs can be found in all existing ARQ protocols. A user data PDU contains at least user data and a sequence number. An error recovery control data PDU contains various amounts of control information needed for error recovery, and control functions such as positive and negative acknowledgments. A common control data PDU contains common control data. Notably, PDUs that include user data and at least a sequence number are denoted herein as Data-PDUs (D-PDUs), and PDUs that include control data needed for error control/recovery are denoted herein as Status-PDUs (S-PDUs).
In the known High Level Data Link Control (HDLC) protocol, which forms the basis for many existing ARQ protocols, the three types of PDUs are called, respectively, information frames (I-frames), supervisory frames (S-frames), and unnumbered frames (U-frames). The RLC protocol used in the existing General Packet Radio Service (GPRS) and the so-called 3rd Generation Cellular Communication System is an example of an HDLC-derived ARQ protocol.
In most communication systems, user data information is conveyed in both directions between the peer entities. A common feature included in an ARQ protocol is that is possible to include error control information in user data PDUs. This capability is known as “piggybacking”. For example, an acknowledgment is included in all I-frames (i.e., D-PDUs) of HDLC-derived protocols. The acknowledgment informs the peer entity about the sequence number of the last (in-sequence) correctly received PDU.
The 3rd Generation Partnership Project (3GPP™) has produced an RLC Protocol Specification for the Radio Access Network (RAN) in the so-called 3rd Generation Digital Cellular Telecommunications System. This system is also known as the Universal Mobile Telecommunication System (UMTS), the UMTS Terrestrial Radio Access (UTRA) system, and the International Mobile Telecommunications-2000 (IMT-2000) system. As such, in accordance with the RLC Protocol Specification for the 3rd Generation System, the RLC sublayer provides three, different data transfer service modes (modes for services that the RLC layer provides to the higher layers): (1) transparent data transfer; (2) unacknowledged data transfer; and (3) acknowledged data transfer. The transparent data transfer service transmits higher layer PDUs to a peer entity without adding any protocol information to these PDUs. The unacknowledged data transfer service transmits higher layer PDUs to a peer entity, but without guaranteeing delivery to the peer entity involved.
The acknowledged data transfer service provided by the RLC Protocol transmits higher layer PDUs to a peer entity with guaranteed delivery. If the RLC sublayer is unable to deliver such data correctly (e.g., error-free delivery), the RLC user at the transmitting side is so notified, and that data is retransmitted. As such, in accordance with the RLC protocol, the acknowledged data transfer mode provides error-free delivery (ensured by retransmission). In other words, the receiving RLC peer entity delivers only error-free SDUs to the higher layer. The acknowledged data transfer mode also provides unique delivery (the RLC sublayer delivers an SDU only once to the receiving upper layer), and in-sequence and out-of-sequence delivery (the RLC sublayer delivers SDUs to the receiving higher layer entity either in the same order or in a different order than what the transmitting higher layer entity submits to the RLC sublayer).
A significant problem with the existing RLC protocol is that only one protocol configuration is specified. Consequently, this protocol is not readily adaptable to the relatively large number of different service situations that can occur in existing and future multi-service systems. However, as described in detail below, the present invention successfully resolves this problem and other related problems.